The students were from Seoul's Ansan Danwon High School. They left from the port city of Incheon, just west of Seoul, for a four-day trip to Jeju. The resort island is considered the Hawaii of Korea.

Around 9 a.m. local time, the ferry sent out its first distress call. It had started to list.

A rescued student, Lim Hyung Min, told CNN affiliate YTN that he heard a loud bump. Several off his classmates were flung off their feet. Everyone was ordered to don life jackets and jump, he said.

Lim said he jumped into the sea before swimming to a rescue vessel.

"I had to swim a bit to get to the boat to be rescued," he said. "The water was so cold and I wanted to live."

Soon, the 6,800-ton ferry sank. Only its white and blue hull remained above water.

A desperate dash

With the clock ticking, rescue crews dashed desperately to get to passengers.

At least 178 people are involved in the rescue work, including 118 marines, 42 navy personnel and 18 police officers, Kang said.

The U.S. Navy ship USS Bonhomme Richard joined the operation and was headed to the scene. It sent helicopters to the area, the U.S. Navy said.

Dive teams have been going in and out of the submerged ship looking for bodies. But poor visibility has made their task difficult.

At Ansan Danwon High School, parents clutched their cell phones in an agonizing wait for a call from their children. Officials posted a list of names. Once a confirmation of a rescue came, they circled that name. So far, 78 students had been pulled to safety.

At one point, the school announced that all students had been rescued but soon backtracked, to the parents' wrath.

Passenger Kim Seung Mok said that despite his efforts and those of others, he couldn't get to several passengers on one of the decks.

"I stayed till the last to rescue people at the hall," Kim told YTN. "But the water was coming in so fast (that) some didn't make it out."